It is known that certain applications, especially in the automotive field, require metal structures to be lightened and have greater strength in the event of an impact, and also good drawability. This requires the use of structural materials that combine high tensile strength with great deformability. In the case of hot-rolled sheet, that is to say with a thickness ranging from 0.6 to 10 mm, these properties are advantageously used to manufacture floor connection parts or wheels, reinforcing parts such as door anti-intrusion bars, or parts intended for heavy vehicles (trucks, buses). In the case of cold-rolled sheet (ranging from 0.2 mm to 4 mm), the applications are for the manufacture of beams that absorb deformation energy or engine cradles, or else skin parts. However, tensile strength and deformability are competing properties, so much so that it is generally not possible to obtain very high values for one of the properties without drastically reducing the other. However, progress has been made recently in trying to meet these requirements better, in particular thanks to the development of what are called TRIP (Transformation Induced Plasticity) steels. However, this type of steel does not make it possible to obtain an elongation of greater than 25% for a strength level of 900 MPa. Although these properties may be satisfactory for a number of applications, they nevertheless remain insufficient if further lightening is desired, and under severe stressing conditions such as those encountered in automobile collisions.
Also known are austenitic Fe-C(0 to 1.5%)-Mn(15 to 35%)-Cr(0 to 20%)-Al(0.1 to 10%)-Si(0 to 4%) steels that combine good strength with excellent ductility. The mode of deformation of these steels depends only on the stacking fault energy or SFE. Among these modes, mechanical twinning makes it possible to obtain high work-hardenability. Twins, by acting as an obstacle to the propagation of dislocations, thus help to increase the flow stress. The twinning deformation mechanism is favored by increasing the stacking fault energy up to a limit (about 30 mJ/m2), above which perfect dislocation glide becomes the dominant deformation mechanism. The SFE increases with the carbon, manganese and aluminum contents. Patent EP 0 573 641 discloses a hot-rolled or cold-rolled austenitic steel containing less than 1.5% C, 15-35% Mn and 0.1-6% aluminum, the strength of which is greater than 490 MPa and the elongation greater than 40% at room temperature.
However, rolling this type of composition requires particular precautions to be taken so as to prevent the formation of defects.
There is also an unsatisfied need for having steel sheet exhibiting even more favorable (strength/elongation at fracture) combinations, while limiting the content of expensive alloying elements.
Furthermore, experience shows that, despite favorable elongation values in uniaxial tension, cold forming (drawing, relatively complex bending, etc.) may pose difficulties in certain cases. In addition, since the parts produced from such sheet very often include regions corresponding to stress concentrations, there is a major need to have steel of high toughness, that is to say in which the fracture or tear resistance in the presence of defects is high, in particular under dynamic stressing. This property is all the more important to take into consideration when the applications of these grades, for example in automobiles, relate specifically to very highly stressed regions and/or to safety components.